In a pressure drop detecting apparatus of the electromagnetic induction type, the resonator assembly mounted on a tire-and-wheel assembly or, particularly, the wheel rim forming part of the tire-and-wheel assembly comprises a pressure-sensitive switch unit which is arranged to be responsive to the pressure of the compressed air in the tire chamber and which is electrically connected between a resonant inductor and a resonant capacitor. The resonance circuit thus constituted by the resonant inductor and capacitor electrically connected together across the pressure-sensitive switch unit is responsive to the electromagnetic wave radiant from the oscillator assembly when the tire-and-wheel assembly rotating about the center axis of the wheel rim assumes a predetermined angular position with respect to the oscillator assembly which is held stationary with respect to the vehicle body structure. In response to the electromagnetic wave emitted from the oscillator assembly, an electric current is induced in the resonance circuit if the pressure-sensitive switch unit intervening between the resonant inductor and capacitor constituting the resonance circuit is closed. The pressure-sensitive switch unit is adapted to be closed in response to an air pressure higher than a predetermined acceptable value and to be open in response to an air pressure lower than the predetermined acceptable value. When the pressure of air in the tire-and-wheel assembly carrying the resonator assembly is higher than the predetermined acceptable value, a current is produced in the resonator assembly each time the tire-and-wheel assembly makes a full turn about the center axis of the wheel rim during cruising of the vehicle. The induction of a current in the resonator assembly results in a normal decrement in the energy of the electromagnetic wave radiant from the oscillator assembly, which thus produces a signal representative of such a normal decrement. The signal produced by the oscillator assembly is delivered to a signal processing unit connected to suitable output means such as a display or warning device and holds the display or warning device inoperative. In the event the pressure of the air in the tire chamber drops unusually due to, for example, a puncture of the tire, the pressure-sensitive switch unit incorporated in the resonator assembly is made open so that the resonant inductor and capacitor of the resonator assembly are electrically disconnected from each other and are disabled from producing a current in response to the electromagnetic wave radiant from the oscillator assembly. As a consequence, the oscillator assembly produces a signal indicative of the elimination of a normal decrement in the energy of the electromagnetic wave from the oscillator assembly and causes the signal processing unit to actuate the display or warning device to produce a sound or luminant signal.
In a prior-art pressure drop detecting apparatus of the electromagnetic induction type such as, for example, the apparatus taught in Japanese Patent Publication No. 50-20436, the pressure-sensitive switch unit is mounted, together with the associated resonant inductor and capacitor, on the wheel rim of a tire-and-wheel assembly and is arranged to be in communication with the tire chamber through a tube connected to the air injection valve attached to the wheel rim. In another known pressure drop detecting apparatus of the electromagnetic induction type such as the apparatus disclosed in Japanese Utility Model Publication No. 49-38275, the pressure-sensitive switch unit and the air injection valve of a tire-and-wheel assembly are constructed as a single structure and the resonant inductor and capacitor mounted on the wheel rim of the tire-and-wheel assembly are electrically connected across the switch unit by insulated wires which are exposed externally with no protective means provided therefor.
In prior-art pressure drop detecting apparatus of these natures, the tube interconnecting the pressure-sensitive switch unit and the air injection valve of the tire-and-wheel assembly or the insulated wires interconnecting the resonant inductor and capacitor across the pressure-sensitive switch unit is supported, without being protected, on the wheel rim of the tire-and-wheel which is subjected to serious weather and operating conditions during use of the tire-and-wheel assembly. For this reason, the resonator assembly of such a pressure drop detecting apparatus is liable to be damaged or fail earlier and is subject to various external disturbances which deteriorate the reliability of operation of the detecting apparatus per se.
Since, furthermore, the pressure-sensitive switch unit and the resonant inductor and capacitor of the resonator assembly of a prior-art pressure drop detecting apparatus of each of the above described natures are disposed separately of each other on the tire-and-wheel assembly, the apparatus as a whole has a disproportionately cumbersome and space-taking construction and the electrical coupling between the resonant inductor and capacitor tends to be considerably unstable.
The present invention contemplates elimination of these and other drawbacks which have been inherent in prior-art tire pressure drop detecting apparatus of the described characters.
It is, accordingly, an important object of the present invention to provide a tire pressure drop detecting apparatus featuring an improved resonator assembly in which the pressure-sensitive switch unit and the resonant inductor and capacitor are arranged compactly on a tire-and-wheel assembly and are reliably protected from the serious weather and operating conditions to which the tire-and-wheel assembly is to be subjected when in use.
It is another important object of the present invention to provide a tire pressure drop detecting apparatus which is reliable in operation and which will provide a prolonged service life.
It is still another important object of the present invention to provide a tire pressure drop detecting apparatus featuring a resonator assembly in which the resonant inductor and capacitor forming part of the resonator assembly are electrically coupled reliably and which can be resonantly coupled with an oscillator assembly efficiently and effectively when the apparatus is in use.
It is, yet, still another important object of the present invention to provide a tire pressure drop detecting apparatus featuring an improved resonator assembly which is easy and economical to manufacture and to install.